1. Field of the Invention
The present invention relates to a manufacturing method of synthetic silica glass.
2. Discussion of the Related Art
Currently, steppers are being used for photolithography to expose and transcribe very small integrated circuit patterns on, for example, semiconductor substrates during the manufacturing process of semiconductor devices. Recently, due to a higher integration of Large Scale Integrated Circuits (LSI), ultraviolet light, which has shorter wavelengths than visible light, has been used in the stepper's light source. Therefore, an optical system of an exposure apparatus must include components that transmit light in the ultraviolet range, since conventional optical glass is impractical to use in such applications. One known example of an optical material with a high transmittance in the ultraviolet range is silica glass.
In addition, the optical system of the exposure apparatus includes multiple optical members, such as lenses, which are used for adjusting for aberrations. Therefore, in order to have a high transmittance for the entire optical system of the exposure apparatus, each individual optical member must have a high transmittance. In order to increase the transmittance of silica glass, the silica glass must have a high purity. One known manufacturing method by which high purity silica glass can be obtained is a flame hydrolysis method (sometimes called a "direct method" or a "direct flame hydrolysis method").
For the flame hydrolysis method, a silicon compound with high purity, such as silicon tetrachloride (SiCl.sub.4) is used. This compound, along with a combustion gas and a combustible gas (such as oxygen and hydrogen), which are used for heating and for the hydrolysis reaction, are expelled from a burner toward a target in a synthesis furnace. The target is rotated and lowered in the synthesis furnace. The starting material expelled from the burner is hydrolyzed by the oxygen/hydrogen flame and forms minute silica glass particles (soot). The soot is deposited, fused, becomes transparent, and forms an ingot of silica glass. The silica glass thus obtained is called synthetic silica glass.
The higher the chlorine concentration in the synthetic silica glass, the lower the durability to ultraviolet radiation of the synthetic silica glass. Therefore, in order to lower the chlorine concentration in the synthetic silica glass, it is preferable to use chloride-free silicon compounds.
When silicon chloride compounds are used, hydrogen chloride, which is a corrosive gas, is generated in the synthesis furnace. To avoid generating hydrogen chloride, it is preferable to use silicon compounds, which are not chlorides, as a starting material for synthetic silica glass.
An example of a technology for using chloride-free organic silicon compounds as a starting material for synthetic silica glass is disclosed in "Tokukaihei" (publication of unexamined patent application) Heisei 4-270130 (1992) (corresponding to U.S. Pat. No. 5,043,002).
Compared to the boiling point of SiCl.sub.4, which is 58.degree. C. to 59.degree. C., the boiling point of many organic silicon compounds is 100.degree. C. or more, due to their high molecular weight. However, heat resistance of even the heat-resistant versions of commercial mass flow meters used for gases, is, at most, only 80.degree. C. Therefore, it has been difficult to control the amount of the gaseous organic silicon compound introduced into the synthesis furnace.
Therefore, there is a need for a manufacturing method for synthetic silica glass that can control the amount of a silicon compound having a high boiling point when introduced to the synthesis furnace.